Er... To put everything on the primary monitor: make everything on top or floating and do as devilboi says. If you have stuff on bottom then this only works when no monitors are in negative space. (unless you use an old desktop module)

Actually, at least in recent builds, $resolutionx$ and $resolutiony$ are the size of the primary monitor, so you can use $resolutionx-100$ instead of -100. (with my mm build, $allresolutionx$ = 1824, but $resolutionx$ = 1024 and I don't mess with that...)

Also, lsbox doesn't support multimonitor at all (afaik), so if you load everything into lsboxes, it will only and always show up on the primary monitor. (hm, maybe not if you put them on bottom, can you do that?) I think this would be the easiest way to have everything stay on the primary monitor, but it's a little restrictive.

Brickle: most modules need to be slightly modified to work properly with multimonitor systems. The current mishmash of some modules compensating properly or improperly for different parts of the whole thing makes it really complicated. I have a build of indiestep that adds per-monitor variables which makes it mostly possible with unmodified modules, but far from easy.